Residential Foundation and Overwintering

[SteveMatt]

Hi folks,

I am planning to build a 24x32 accessory structure, for myself. One of the gable ends will be below grade (i.e., a walkout)

Location is northern Vermont.

For budget reasons, it will be a multi-year project.

I would like to phase it such that the site work, footings, and foundation are one phase, in one year. Given that, my general thoughts and plan for this phase are:

1. place footings and footing drains;
2. get the walls in;
3. insulate the outside of the walls with "2 rigid foam board (foil faced for vapor barrier);
4. backfill with crushed stone to finished grade height (i.e., 3-4' wide curtain-like around the perimeter);

Given that approach, I would like to better understand the risks of the footings and walls and how to mitigate them. I understand the risk frost poses. So I am looking for best-practice suggestions for keeping the concrete protected over the winter (e.g., layering 2" poly foam board or hay bales along the interior of the walls, etc.)

Thanks,
Steve

 

[jayrod12]

Adding insulation around the insulation on both sides will delay the frost, but it won't stop it outright if there isn't a source of heat on one side of the wall. I'm not sure what kind of frost depths and average low temperatures we're discussing, but I'd be hesitant to do this. I'd prefer, if it were me, to frame the floor above as well, making the basement a closed box and provide enough heat in there to keep it above freezing. Then I'd be making sure that the outside of the wall has insulation extending away from the foundation to delay the frost penetration on the outside and the heat loass through the fround from the inside should prevent frost formation in the supporting soils.

 

[oldestguy]

To clarify what jayrod says. I have had two occasions for this situation on my own places One was done the usual way setting the foundations to the code depth along that part of the wall. The other was my current house with a half height wall, the rest buried 4 feet. Installing a walk out put that new cold space down just above footing grade. I placed closed cell foam 3 inches thick (the kind that takes pressure) under the floor slab that also carries the new entry walls. The outsides of the new entry walls had 2" closed cell foam. If you have the chance, first bury a 4' wide sheet of the closed cell foam a foot or so under the grade. In my consulting business I have recommended this many times and no complainants resulted. In addition check the ground type there. If the P-200 sieve sizes is less than 5% (clean sand), you probably have no frost heave. I have recommended (and used myself) replacing the original ground down 4 feet with this clean sand, compacted. Extend the treatment, 4 feet out at frost depth. This in central Wisconsin.

 

[darthsoilsguy2]

walkouts are usually frost wall construction on the walk-out side with a shallow foundation on the buried concrete wall side. the frost wall side should be fine if backfilled to finished subgrade but the risk is that the footing on the non-walkout side would probably be well above frost depth (which is 5 ft where i work on the south end of VT) and not be protected. You don't want the soils beneath your footing to have a freeze/thaw cycle or it opens you up to a lot of uncertainties. There are lots of ways to mitigate the risk by insulating or soil cover. If all the paths to get to the bottom of footing have pass through an equivalent of R15, you should be ok. But, how can budgeting be putting you in this situation. It is March and construction season doesn't even really kick in for another month. The concrete foundation work can be done in a few weeks to a month... why not just put it off until you can go full steam ahead? or maybe just work on your site prep work this year. Your question is more of a September question for a job that is already underway. i guess this makes sense if you have some start-by-date constraint you aren't telling us about

 

[SteveMatt]

Guys,

Thanks for your inputs, very much appreciated. Yes Darth, it actually is more of a September question because that's when we'd likely be getting to this unfortunately. Just to clarify too (I'd forgot to mention) that there will FOAMULAR 1000 XPS under the footings extending 4' to the outside of the footings. And eventually, use their 400 version for under th finished floor.

In terms of timing, I agree that if I can get the deck on, it would be the best situation. Because it would allow me to backfill (crush stoned) to finished grade around 3 sides, the 3 walls will tied together, and mitigate frost issues in those areas...leaving only the walkout side to protect.

My thinking and planning is now looking like - if there is schedule risk that I cannot get all the way to the deck, then as darth suggests I will only do sitework this year. Thanks again.

-Steve

 

[darthsoilsguy2]

the main risks are soil-related with frost jacking laterally against the walls for unbalanced fills or messing up your bearing soils.... your question was about the concrete itself.... if the soils truly are all squared away, the concrete itself is pretty much self-protecting if you have air entrainment in the 6% region and at least a 4000 psi mix, which you should have been doing anyway for this climate. although the concrete would protect itself... consider laying concrete curing blankets over the wall and weighing them down on both sides for the down-time... it wouldn't hurt, will provide some additional frost penetration insurance, and would help keep the construction cleaner for when you return

 

[SteveMatt]

thanks Darth!